1. Field of the Invention
This invention relates to methods for the detection and identification of uranium bearing materials in media, in the form found in nature such as in rocks, soils, clays, streams and lakes, decayed organic matter, vegetation, subsurface water, and films or in media which contain uranium compounds or have come in contact with uranium compounds such as products and effluents from installations, processing or utilizing uranium compounds such as in elemental or isotopic separation, purification, recovery and ore beneficiation. One method utilizes a time-delayed measurement, in one or more narrow spectral ranges, of the intensity and decay time of the luminescence emitted by hexavalent uranyl ions when irradiated by light from a source having a wavelength in the range of about 2400 to 5000 A for a duration equal to or less than about 100 microseconds.
2. Prior Art
When certain materials are irradiated with ultraviolet light, i.e. light having a wavelength in the range of about 1800 to 3700 A, and after cessation of the irradiation, photons are emitted from the material, the material is said to exhibit luminescence. By definition in the prior art, if the luminescence ceases on termination of the irradiation, the phenomenon is termed fluorescence; if the luminescence persists after termination of irradiation, the phenomenon is termed phosphorescence. These definitions were made at a time when observations of the persistence of luminiscence was made with the unaided eye, but with the development of instruments which can measure the persistance of luminescence in nanoseconds after termination of irradiation, it is now known that many materials, in particular, uranium-bearing substances comprising hexavalent uranyl ions which have been characterized in the literature as fluorescent substances, emit luminescence for as long as 1000 microseconds after termination of light excitation.
Since the term "fluorescence" and "phosphorescence" have been standardized in the literature for more than 100 years, the more general term "luminescence" will be used herein to describe the emission of light from a material after termination of light excitation.
Deposits of uranium minerals, for example originating from igneous rocks or volcanic tuff which have eroded, are solubilized and carried by oxidizing surface waters to lower strata where they are redeposited, for example, in sandstones by reducing agents such as pyrite, hydrogen sulfide, or hydrocarbons. Where such deposits have surface expressions and have been reoxidized in part, ordinary methods for prospecting in the field for uranium minerals have depended in the past on the luminescence that is observed when these minerals are illuminated with ultraviolet light from a portable lamp.
A major difficulty in the use of this method is that the terrain must be readily accessible, and a great many nonuranium bearing minerals, organics and other natural substances luminesce when excited in the same spectral range as uranium-bearing substances or have luminescence wavelengths which are common to the emission from uranium-bearing substances and thus obscure their luminescence, especially when these substances are present in low concentrations in the material under examination.
More recently, remote detection of substances which luminesce has been proposed by means of Fraunhofer line-depth discrimination using sunlight and air-borne LIDAR (See: W. R. Hemphill et al, 6th Sympos. on Remote Sensing, Ann Arbor, Mich. Oct. 19, 1969); Continuous-Wave Laser Flurometer (See O'Neil et al, Water Science-Inland Waters Directorate, Canada). In addition, U.S. Pat. No. 3,961,187 discloses a method for detecting oil seepages at sea from an aircraft using a laser beam, and the UA-3 uranium analyser marketed by Scintrex Limited of Canada uses a laser in the ultraviolet range and an undisclosed proprietary reagent for the determination of uranium in a water sample contained in a quartz cell.